1. Field of Invention
This invention relates generally to the measurement or quantification of a volume of liquid, especially as used in personal hydration systems for individuals for carrying and supplying drinking water and electrolyte replacement fluids.
2. Prior Art
Personal hydration systems generally comprise a flexible bladder and a drinking tube, and are usually carried in a backpack or waist pack. A product popular with hikers, cyclists and skiers is sold under the trademark CamelBak, by CamelBak Products, Inc. of Petaluma, Calif. Numerous competitors offer similar products. To prevent leakage, the bladder is usually not vented and therefore collapses upon itself as the fluid is withdrawn. A flexible tube connects to the bladder and enables the user to withdraw fluid by creating suction on the other end of the tube. There is usually a mouthpiece with a valve attached to the tube to facilitate the withdrawal of the fluid into the user's mouth. The unit does not measure the fluid withdrawn, or the amount remaining in the reservoir. The unit generally does not notify the user that the reservoir is about to be depleted until actual depletion occurs.
Hydration bladders have numerous advantages over water bottles and flasks. The flexible drinking tube, usually combined with a bite-actuated mouthpiece valve, makes drinking easier during physical activity. Also, a greater volume and hence weight may be more efficiently carried by distributing the weight over a larger area. Finally, the bladder may be encased in an insulating sleeve, providing some degree of temperature control. However, a major disadvantage of hydration systems as compared to water bottles, is the monitoring of fluid levels. Water bottles may be visually inspected and/or shaken to easily confirm how much fluid remains as they are used. The hydration bladder is usually located on the individual's back, often deep inside of a backpack and obscured by other items in the backpack. The user of a hydration bladder typically has no warning that he is about to run out.
Knowledge of the volume of fluid remaining in the bladder is important for several reasons. First, it is crucial that the body remain properly hydrated, especially during physical activity. Knowing that the bladder was running low would enable the individual to refill it in a timely way, preserving her health and well being. Additionally, in the case of backpacking in wilderness areas, quite often there is a limited availability of sources for water. Since such sources often do not contain safe drinking water, the water must be filtered or treated with chemicals prior to drinking. Both these activities result in a time period elapsing before the water may be consumed. It would be a great advantage if an individual could easily know when the bladder had been depleted to a certain predetermined level or levels.
Prior-art methods of indicating fluid levels in containers, such as floats and optical proximity sensors are well documented. Chai, in U.S. Pat. No. 5,341,686 (August, 1994) teaches a vertical water meter, but Chai's meter is effective when used with rigid containers having a stable shape. A hydration bladder is a dynamic object whose geometry changes as fluid is withdrawn or added. Additionally, hydration bladders are not in a constant vertical position due to their being attached to the body of the user, and are subject to many degrees of motion. Further complicating this is the bladder's susceptibility to external pressure from other items loaded into a backpack.
Griffiths, et al., in U.S. Pat. No. 4,350,265 (September, 1982) teaches a liquid dispenser meter with for a liquid dispenser having an axially movable plunger. The operation of a personal hydration bladder is controlled by the user's mouth and the amount of liquid withdrawn naturally varies significantly, making Griffiths' liquid dispenser unsuitable.
Inline flow metering systems, such as taught by Perkins, U.S. Pat. No. 6,212,959 (April, 2001), can provide an indication of fluid volume, but only if the user manually calibrates or otherwise updates the system whenever fluid is put into the bladder. Then the system can deduct the amount of fluid withdrawn and provide an indication of remaining fluid. This method is impractical because bladders are often used in the outdoors and refilled or partially refilled from natural water sources, through filter/pump systems, or from additional containers of fluid, the exact volume of which may not be known. Perkins' meter system is “blind” to the amount added to the bladder and therefore cannot provide any warning as to when the bladder will empty. Additional disadvantages to flow meter systems are complexity, high cost, and failure potential from sub-freezing temperatures. Because they use invasive sensing, i.e., devices immersed in the fluid, and must be held to tight tolerances due the low flow rates encountered in this application, flow meter systems are also vulnerable to fouling from nutritional additives and sports drinks, which are often mixed into the water. The interactive hydration prompt feature of flow meter systems has merit, but has limitations in that the system does not account for ambient temperature, relative humidity, actual perspiration loss, and exertion level of the user—all of which are determinants of optimal re-hydration rates.
Magnetically actuated reed switches are well known in the prior art. They comprise a pair of low-reluctance ferromagnetic reeds that overlap at their free ends (the contact area). When a magnetic field is brought into proximity with the reeds, the extreme ends of the reeds become magnetized, each assuming the opposite magnetic polarity. When the field becomes strong enough, i.e., the magnet is at the desired proximity, the overlapping ends attract and join together, completing the electrical circuit. When the field is removed, the reeds lose their magnetic properties and separated through their own spring tension, breaking the electrical circuit.
Angular Displacement Sensors are well known in the prior-art as devices which can detect angular displacement or bending. Gentile, et al, U.S. Pat. No. 5,086,785 (February, 1992) discloses a substrate, which may be attached to an object and will change its resistance as it is bent.
Additionally, the weight of a device used for the purpose of alerting the user of a personal hydration system to fluid levels is an important consideration. Since the user will carry it, it is essential that the weight be kept to minimum. The devices discussed above would add an undesirable amount of bulk and weight to the payload carried by the user.